This invention relates to a method of preparing rare earth-iron alloys. More specifically, this invention relates to an improved method of preparing high-purity rare earth-iron binary and ternary alloys by the thermite reduction method.
A number of rare earth-iron alloys have been developed which have interesting physical properties. For example, rare earth-iron alloys having magnetostrictive properties were described by Savage et al. in U.S. Pat. No. 4,308,474 which issued Dec. 29, 1981. The materials described therein were found to be particularly useful in magnetostrictive transducers, delay lines, variable frequency resonators and filters.
Another series of alloys based on the combination of rare earth, iron and boron were described in Materials Letters, Vol. 2, Number 2, October 1983, page 169 et seq. and in the J. Appl. Phys. 55(6), Mar. 15, 1984, page 2078 et seq. Nd-Fe-B and Pr-Fe-B alloys were described which show great promise as permanent magnet materials.
These alloys are expensive because of the cost of purifying the starting materials and the number of steps required to prepare these materials. Typically, the alloy is prepared by melting together the several purified metals which will constitute the alloy. The difficulty, however, arises in the preparation of high-purity rare earth metals. For example, a terbium-dysprosium-iron alloy may be prepared by first fluorinating terbium oxide with hydrogen fluoride to form terbium fluoride (TbF.sub.3). The terbium fluoride is then reduced with calcium metal to form an impure terbium metal. This terbium is then purified by heating to 1600.degree. to 1700.degree. C. to sublime the metal away from the impurities, condensing it on a cold head. The sublimed metal is then arc melted to form a bar. Using the same series of steps, high purity dysprosium metal is separately prepared and formed into a bar. Only at this time can appropriate quantitites of the purified terbium metal, dysprosium metal and purified iron be arc melted together to form the terbium-dysprosium-iron alloy.
As the example illustrates, the preparation of an alloy is time consuming and requires a substantial amount of energy, both of which go to increase the cost of preparing such rare earth-iron alloys.
Furthermore, it should be noted that in preparing pure un-alloyed rare earth metals using metallothermic methods, extreme care must always be taken to insure that oxygen, nitrogen, and carbon contamination does not occur during processing. The rare earth metal has a high affinity for these impurities and they can greatly effect the properties of the rare earth metals.
An improved method for the preparation of high-purity rare earth-iron alloys has been developed by which the alloys can quickly and economically be prepared by thermite reduction of rare earth and iron fluorides.